A magnetic memory or a magnetic random access memory (MRAM) is a nonvolatile memory capable of a high-speed operation and rewriting an infinite number of times. Therefore, some types of MRAMs have been put into practical use, and some types of MRAMs have been developing to improve their general versatility. In the MRAM, magnetic material is used for a memory element, and data is stored in the memory element as a magnetization direction of the magnetic material. Some methods for switching the magnetization direction of the magnetic material are proposed. Those methods have in common with usage of a current. To put the MRAM into practical use, it is important to reduce this writing current as much as possible. According to a non-patent literature 1, it is required that the wiring current should be reduced to be equal to or less than 0.5 mA, preferably equal to or less than 0.2 mA. This is because the minimum layout can be applied to the 2T-1MTJ (Two transistors—One Magnetic tunnel junction) circuit configuration proposed in the non-patent literature 1 to realize the cost performance equal to or more than that of the existing volatile memory such as a DRAM and a SRAM and so on.
The most general method of writing data in the MRAM is to switch a magnetization direction of a magnetic memory element by a magnetic field which is generated by passing a current through a wiring line for a writing operation prepared on the periphery of the magnetic memory element. Since this method uses the magnetization switching caused by the magnetic field, the MRAM can theoretically perform the writing at the speed of 1 nano-second or less and thus, the MRAM is suitable for the high-speed MRAM. However, a magnetic field for switching magnetization of a magnetic material securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such a magnetic field, the writing current of about a few mA is needed. In this case, a chip area is necessarily large and power consumed for the writing increases. Therefore, this MRAM is not competitive with other kinds of random access memories. In addition, when the size of the memory cell is miniaturized, the writing current further increases and is not scaling, which is not preferable.
Recently, as methods to solving these problems, following two methods are proposed. The first method is a method using a spin transfer magnetization switching. This method uses a lamination layer including a first magnetic layer (magnetization free layer) which has magnetization that can be switched, and a second magnetic layer (reference layer) which is electrically connected to the first magnetic layer and has fixed magnetization. In the method, the magnetization in the first magnetic layer (magnetization free layer) is switched by using an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer (magnetization free layer) when a current flows between the second magnetic layer (reference layer) and the first magnetic layer (magnetization free layer). A reading operation is carried out by using a magnetoresistive effect generated between the first magnetic layer (magnetization free layer) and the second magnetic layer(reference layer). Therefore, the MRAM using the spin transfer magnetization switching method is an element having two terminals. The spin transfer magnetization switching is generated when a current density is equal to or more than a certain value. Accordingly, as the size of the element decreases, the writing current is also decreased. In other words, the spin transfer magnetization switching method is excellent in scaling performance. However, generally, an insulating film is provided between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer) and a relatively large current should be made to flow through the insulating film in the writing operation. Thus, there are problems regarding a writing resistance property to and reliability. In addition, there is concern that a writing error occurs in the reading operation because a current path of the writing operation is the same as that of the reading operation. As mentioned above, although the spin transfer magnetization switching method is excellent in the scaling performance, there are some obstacles to put it into practical use.
On the other hand, the second method, which is a magnetization switching method using a current induced domain wall motion effect, can solve the above-mentioned problems that the spin transfer magnetization switching method is confronted with. For example, a MRAM using the current induced domain wall motion effect is disclosed in a patent literature 1. Regarding the MRAM using the current induced domain wall motion effect, generally, in the first magnetic layer (magnetization free layer) having the magnetization which can be switched, magnetization of both end portions is fixed such that the magnetization of one end portion is approximately anti-parallel to that of the other end portion, and magnetization of the center portion can be switched. In the case of such magnetization arrangement, a domain wall is introduced into any of ends the center portion in the first magnetic layer. Here, as reported in a non-patent literature 2, when a current flows in a direction where the current penetrates the domain wall, the domain wall moves in the direction same as the direction of the conduction electrons in the center portion. Therefore, the data writing can be realized by making the current flow inside the first magnetic layer (magnetization free layer). The data reading is realized by using the magnetoresistive effect caused by a magnetic tunnel junction (MTJ) provided in a region (the center portion) where the domain wall moves. Therefore, the MRAM using the current induced domain wall motion method is an element having three terminals, and fits in the 2T-1MTJ configuration proposed in the above-mentioned non-patent literature 1. The current induced domain wall motion is generated when a current density is equal to or more than a certain value. Thus, this MRAM has the scaling property similar to the MRAM using the spin transfer magnetization switching. In addition, in the MRAM element using the current induced domain wall motion, the writing current does not flow through the insulating layer in the magnetic tunnel junction and the current path of the writing operation is different from that of the reading operation. Consequently, the above-mentioned problems caused in the spin transfer magnetization switching can be solved.
Meanwhile, in the non-patent literature 2, a current density of approximately 1×108 A/cm2 is required for the current induced driven domain wall motion. For example, it is assumed that a width and a thickness of a layer where the domain wall motion arises are 100 nm and 10 nm, respectively. In this case, the writing current is 1 mA. This cannot satisfy the above-described condition for the writing current. However, as described in a non-patent literature 3, it is reported that, by using material having perpendicular magnetic anisotropy for a ferromagnetic layer (magnetization free layer) where the current induced domain wall motion arises, the writing current can be sufficiently reduced. Because of this, in the case of manufacturing the MRAM using the current induced domain wall motion, it is preferable to use ferromagnetic material having perpendicular magnetic anisotropy as a layer (magnetization free layer) where the domain wall motion arises.
As a related art, Japanese patent application publication JP2009-252909A discloses a magnetoresistive effect element and a magnetic random access memory. This magnetoresistive effect element includes a first ferromagnetic layer composed of ferromagnetic material with perpendicular magnetic anisotropy. The first ferromagnetic layer includes a first magnetization fixed region, a second magnetization fixed region and a magnetization free region. The first magnetization fixed region has magnetization fixed in a first direction. The second magnetization fixed region has magnetization fixed in a direction anti-parallel to the first direction. The magnetization free region is connected with the first and second magnetization fixed regions and has magnetization which can be switched. An upper surface of one of the first magnetization fixed region and the magnetization free region is higher than an upper surface of the other in a substrate-perpendicular direction. A lower surface of one of the first magnetization fixed region and the magnetization free region is lower than a lower surface of the other in the substrate-perpendicular direction.
International patent publication WO2005/069368A (corresponding to US patent application publication US 2008/137405(A1) discloses a current injection domain wall motion element. This current injection domain wall motion element has fine junctions of first and second magnetic bodies which have magnetization directions anti-parallel to each other and a third magnetic body sandwiched the first magnetic body and the second magnetic body. By making a current flow across boundaries of the fine junctions, an interaction between the current and a domain wall causes the domain wall to move toward a current direction or a direction opposite to the current direction, thereby controlling a magnetization direction of the element.
Japanese patent application publication JP2006-270069A discloses a magnetoresistive effect element and a high-speed magnetic recording device based on a domain wall motion using a pulse current. This magnetoresistive effect element includes a first magnetization fixed layer/a magnetization free layer/a second magnetization fixed layer. The magnetoresistive effect element includes a mechanism for inducing a domain wall generation in a transition region between the magnetization fixed layer and magnetization free layer, the transition region being at least one of a boundary between the first magnetization fixed layer/the magnetization free layer and a boundary between the magnetization free layer/the second magnetization fixed layer. The magnetization directions of these magnetization fixed layers are set to approximately anti-parallel magnetizations. The domain wall exists one of the transition regions between the first magnetization fixed layer/the magnetization free layer and between the second magnetization fixed layer/the magnetization free layer. By applying a current equal to or less than 106 A/cm2 with a certain pulse width, the domain wall moves between two transition regions, thereby making the magnetization of the magnetization free layer switch and detecting the magnetoresistive value caused by the switching of the relative magnetization direction.
Japanese patent application publication JP 2007-103663A1 discloses a magnetic element, a recoding reproducing element, a logic operational element and a logic operational device. The magnetic element includes a first magnetic layer, a non-magnetic layer and a second magnetic layer. The first magnetic layer includes a magnetization changeable region in which magnetization can be changed to one of a first direction and a second direction anti-parallel to the first direction and a first electrode which introduces a current into itself inside. The non-magnetic layer has contact with the magnetization changeable region of the first magnetic layer at its surface and includes a second electrode which applies a certain potential to itself. The second magnetic layer has contact with a back surface of the non-magnetic layer, has its internal magnetization which is previously fixed in one of first and second directions, and includes a third electrode which detects its potential.